﻿76 



PROCEEDINGS OF THE 



[Dec, 1856. 



is the first tolerably distinct indication of lobation in the outstand- 

 ing fold or disk. A division of it into eight peripheral lobes, each 

 of which is so placed as to have an otolithic cyst at its rounded 

 outer apex, and between every two of which is the base of one of 

 the eight muscular areas hereafter to be described, is the last stage 

 at present known to me. A further development of these lobes 

 next takes place, the marginal cord which has heretofore appeared 

 nearly straight in its course, is now bent out at the interspaces 

 between the tentacula, giving it, at the same time, the appearance 

 of being looped up towards the base of each tentacle so that the 

 disk is now divided into eight free peripheral lobes, each of which 

 bears at its outer free edge an otolothic cyst, as represented PI. 6, 

 fig. 28, and also at a somewhat more advanced stage, PL 5, fig. 

 28b, at a. In the notches or fissures between these lobes shortly 

 now begins to appear a delicate membrane of different cellular 

 structure from the surrounding parts, which is the rudimentary 

 vail. It doubtless, at the same time, is continuous on the lower 

 edge of each lobe, stretching thus from notch to notch around the 

 whole periphery, but I have not been able to trace it, with cer- 

 tainty. This is visible in the figure last referred to at a. At the 

 same time the cord, represented in the figure by the dark outline 

 of the lobes, is distinctly looped up as it were by each tentaculum, 

 and the disk has assumed in outline all its chief peculiarities. 



Up to this time no great change has occurred in the proboscis or 

 digestive cavity of the larva, except that the former has become 

 enormously elongated, and the latter has followed above the more 

 protuberant outline of the gemmiparous surface, while at the same 

 time the whole digestive bulb has increased considerably in diame- 

 ter, especially in that part surrounded by the Medusa-disk. The 

 process of feeding from the stomach of the mother also still takes 

 place as represented in the figure. But now some slight indica- 

 tions of change in form are visible, in that part of the digestive 

 cavity surrounded by the disk already referred to as having in- 

 creased in diameter. In outline it has been hitherto circular, but 

 it now begins to exhibit faint indications of an octagonal form, 

 each angle of which is at the base of a tentaculum. No further 

 change immediately occurs worthy of especial notice until the 

 larva is nearly ready for freedom. The disk continues to grow, 

 each of the characters just described in their incipience becomes 

 more and more developed, until it presents a sufficient expanse to 

 be used as an umbrella-like swimming organ. The time for expul- 



